In single-ended line tests (i.e. SELT, see, e.g., ITU-T Standard, G.993.2, SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS, Digital sections and digital line system—Access networks, Very high speed digital subscriber line transceivers 2 (VDSL2) (February 2006); ITU-T G.996.2, SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS, Digital sections and digital line system—Access networks, Line Testing for Digital Subscriber lines (DSL); and “Feasibility and problems of DSL loop topology identification via single-ended line tests”, Carine Neus, 16th IMEKO TC4 Symposium Exploring New Frontiers of Instrumentation and Methods for Electrical and Electronic Measurements Sep. 22-24, 2008, Florence, Italy), a known signal is sent over the loop and the reflected signal is analyzed to determine loop characteristics and any impairments present on the line.
One example of a loop impairment is a bridgetap (BT), which is an extraneous dangling cable connected to the line between a customer premises equipment (CPE) and a central office (CO) as a T or a branch. These are typically from configurations that allow the cable company to assign the same main line to different users. A BT on a line between a CO and a CPE causes impedance mismatch and signal reflections leading to a loss in bandwidth capacity on the line. BT's can be a concern to service providers since they typically do not have a historical record of BT locations. If the BT's can be detected and their location and length can be estimated then they can be removed by a technician if required. Hence it would be desirable if SELT could accurately detect bridgetaps and get an accurate loop makeup.
Breaks in a line or line-cuts are additional well-known problems for xDSL systems. These lead to a loss of connectivity and an extensive investigation to fix the problem. Accurately identifying and locating line-cuts using SELT would also be desirable.